A conventional hybrid driving force transmission device in which an engine, a motor and clutch unit and a transmission unit are connected in series is known. The motor and clutch unit includes a multiple-plate dry clutch disposed inside an electrical motor. More specifically, the motor and clutch unit includes a clutch hub connected to an output shaft of the engine, a clutch drum to which a rotor of the electrical motor is fixed and which is connected to an input shaft of the transmission unit, and the multiple-plate dry clutch disposed between the clutch hub and the clutch drum (for example, Japanese Laid-Open Patent Application No. 2010-151313).
However, in the conventional hybrid driving force transmission device, the multiple-plate dry clutch is housed in the dry space covered by a unit housing, and sealed by the sealing member. Therefore, when friction facings are subjected to repeated connections/disconnections, the abrasion powder accumulate between friction surfaces without being discharged to the outside, which would cause drag between drive plates and driven plates with the result that the multiple-plate dry clutch will suffer from poor engagement/release.